1. Field of the Invention
The present invention relates to a method for fabricating porous UO2 sintered pellets for an electrolytic reduction process for recovering metallic nuclear fuel, and porous UO2 sintered pellets fabricated in the same way, and more particularly, to a method for fabricating porous UO2 sintered pellets for an electrolytic reduction process by discontinuously performing atmospheric sintering and reduction to recover the metallic nuclear fuel.
2. Description of the Related Art
Spent nuclear fuel (UO2) from a light water reactor (LWR) generally includes fissile material (U) that is not consumed, and transuranic elements (TRU) that are generated from the burning. Along with this, UO2 also includes fission products. The pyroprocess is a recycle technology implemented to produce metallic nuclear fuel for use in a fast reactor, through pyrometallurgical and electrochemical processing from irradiated UO2 fuel in the LWR, thus providing advantages including good nuclear proliferation resistance. To recover the fissile material, the pyroprocessing mainly includes a pretreatment process to fabricate UO2 sintered pellets from U3O8 powder, and a follow-up process to convert the fabricated UO2 sintered pellets (i.e., ceramic nuclear fuel) into metallic nuclear fuel. The presence of fission products is desirably removed in the pretreatment process in consideration of the considerable influence on the follow-up process where the ceramic fuel is converted into metallic fuel. To be specific, the pretreatment process generally involves disassembly/cutting of a fuel rod, decladding, compacting, and sintering, and the follow-up process mainly involves electrolytic reduction, electro-refining, and electro-winning (see FIG. 1). The decladding in the pretreatment process relates to extracting spent UO2 sintered pellets from the disassembly/cut fuel rod, in which the UO2 sintered pellets within the fuel rod are generally converted into U3O8 in an air atmosphere at temperatures ranging between 350 and 700° C. The UO2 pellets are powdered owing to a volume expansion in accordance with the decreased density, and thus escapes from the fuel rod. As the phase changes from UO2 pellets to U3O8 powder from oxidation, gaseous volatile fission products including iodine (I) and bromine (Br) existing in the pellet are vaporized.
After the decladding, the U3O8 powder is compacted into the desired shapes and dimensions using a compacting machine such as a press. Then, by sintering at the appropriate temperature under desired atmospheric gas (e.g., oxidizing, inactive, nitrogen, and reducing gas), porous sintered pellets are fabricated, and are suitable for a volatilization of the fission products and are suitable for handling. Porous UO2 sintered pellets are advantageous, considering the fact that fission products are easily volatilized, and when the following electrolytic reduction is processed with UO2 rather than U3O8, the O/U ratio is decreased from 2.67 to 2.00, and owing to the decrease in the existing oxygen, the processing efficiency is increased greatly. Further, the process yield is increased, such that there is an advantage of increased productivity.
In a conventional technology, the U3O8 powder is compacted, and sintered for a predetermined time under an oxidizing, inactive, or nitrogen (N2) gas atmosphere, and thus UO2, sintered pellets (not porous UO2) are fabricated. If U3O8 green pellets are sintered for a predetermined time under a reducing atmosphere, it would be possible to fabricate porous UO2 sintered pellets. However, considering the fact that a low sintering temperature even under a reducing atmosphere will result in the fabrication of UO2+x (x=0.01-0.13) sintered pellets having a O/U ratio (i.e., ratio between oxygen elements to uranium elements) other than 2.00, it is necessary that the temperature be at least 1400° C. or greater to ensure that the porous UO2 sintered pellets are fabricated (see FIG. 1). Further, upon observation of the fracture surface of the sintered pellet fabricated under a reducing atmosphere, if the sintering temperature was relatively lower (i.e., lower than or equal to 1200° C.), there were relatively more inter-particle bonded aggregates of the powder, while at relatively higher sintering temperature (i.e., higher than or equal to 1400° C.), there were independently-existing powder particles, and inter-particle bonding was not observed (see FIG. 2). This indicates the fact that, above or equal to 1400° C., U3O8 is completely reduced into UO2, thereby removing inter-particle bonding.
Meanwhile, after U3O8 powder extracted from the fuel rod are compacted into a desired shape (cylindrical or cubical shape) and dimensions using a press, pores suitable for the volatilization of the fission products in the pellet are massively generated during sintering under an atmospheric gas (oxidizing, inactive, reducing, and nitrogen). Owing to the presence of the pores generated as explained above, the semi-volatile fission products existing in the pellet matrix are allowed to be more easily volatilized, and as the atmospheric gas facilitates the volatilization of the fission products, the fission products are basically not remained in the pellet matrix.
Korean Patent No. 10-0293482, incorporated herein by reference in its entirety, teaches a method for fabricating UO2 sintered pellets, which includes steps of fabricating green pellets by adding various kinds of sintering aids into oxidized U3O8 powder transformed from UO2 spent nuclear fuel, and fabricating UO2 sintered pellets by sintering the green pellets at temperatures above or equal to 1500° C. under a reducing atmosphere, thereby providing the advantage of providing UO2 sintered pellets with high sintered density. However, when the sintering under a high-temperature reducing atmosphere above or equal to 1400° is performed, the powder particles are not linked, but exist independently from each other in the fabricated sintered pellets. If this happens, the sintered pellets do not maintain their shape and collapse into fragments in the follow-up process, i.e., the electrolytic reduction. The fragments will then cause additional shortcomings such as inconvenient handling in the follow-up process. Further, the additives, which are added to enhance the sintered density of the sintered pellet, unnecessarily remain to affect the process when the metallic fuel is recovered by electrolytic reduction. Further, since such fuels including additives will also produce undesirable fission products in large amounts when recycled at a later stage, recycling can be inefficient.
In awareness of the above, the present inventors have been investigating a method for fabricating porous UO2 sintered pellets for an electrolytic reduction for the purpose of recovering metallic fuel from the spent nuclear fuel (UO2), and were able to develop a method for fabricating porous UO2 sintered pellets, which involves the steps of oxidizing the spent nuclear fuel (UO2) into U3O8, compacting the result into green pellets, sintering the green pellets to remove volatile and semi-volatile fission products, cooling the result at room temperature, and re-heating the cooled UO2+x sintered pellets for a reduction under a reducing atmosphere, and thus completed the present invention.